US 7441446 B2
By digitizing the UFM signal without using a lock-in amplifier, substantially all of the information initially contained in the UFM output signal may be maintained and may then be used for further data processing. Consequently, any type of model or evaluation algorithm may be used without being restricted to a very narrow bandwidth, as is the case in lock-in based techniques. The digitizing is performed on a real-time basis, wherein a complete UFM curve is digitized and stored for each scan position. In this way, quantitative meaningful values for specific surface-related characteristics with a nanometer resolution may be obtained.
1. A metrology system, comprising:
a scanning probe microscope;
a modulation unit connected to the scanning probe microscope to enable a modulated excitation of a sample;
a signal processing unit connected to said scanning probe microscope to receive an output signal of said scanning probe for a plurality of scan positions defining a two-dimensional region said signal processing unit being configured to digitize and record a frequency spectrum including a plurality of frequency components of a frequency band for each scan position; and
a data processor operable to generate an image of the sample over the two-dimensional region based on the plurality of frequency components of the frequency band.
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11. A method, comprising:
scanning a sample surface by a scanning probe at least a plurality of scan positions defining a two-dimensional region to obtain a surface-related output signal at each of said plurality of scan positions;
exciting said sample surface with a modulated signal;
digitizing said output signal including a frequency spectrum including a plurality of frequency components of a frequency band for each of said plurality of scan positions; and
generating an image of the sample over the two-dimensional region based on the plurality of frequency components of the frequency band.
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1. Field of the Invention
Generally, the present invention relates to metrology in the manufacturing of microstructures, such as integrated circuits, and, more particularly, to the measurement of the surface characteristics of microstructure features by means of SPM (scanning probe microscopy) metrology tools with modulated surface excitation, which allow the determination of surface and near-surface characteristics with nanometer resolution.
2. Description of the Related Art
In manufacturing microstructures, such as integrated circuits, micromechanical devices, opto-electronic components and the like, device features such as circuit elements are typically formed on an appropriate substrate by patterning the surface portions of one or more material layers previously formed on the substrate. Since the dimensions, i.e., the length, width and height of individual features, are steadily decreasing in order to enhance performance and improve cost-effectiveness, these dimensions have to be maintained within tightly set tolerances in order to guarantee the required functionality of the complete device. Usually, a large number of process steps have to be carried out for completing a microstructure, and, thus, the dimensions of the features during the various manufacturing stages have to be thoroughly monitored to maintain process control and to avoid further cost-intensive process steps owing to process tools that fail to meet the specifications in an early manufacturing stage.
For example, in highly sophisticated CMOS devices, the gate electrode, which may be considered as a polysilicon line formed on a thin gate insulation layer, is an extremely critical feature of a field effect transistor and significantly influences the characteristics thereof. Consequently, the size and shape of the gate electrode has to be precisely controlled to provide the required transistor properties. Thus, great efforts are being made to steadily monitor the dimensions of the gate electrode.
However, for highly scaled microstructures, the surface characteristics of the materials used increasingly become important aspects for enhancing performance and reliability of the devices and/or for reducing yield loss and the like. For example, the detection of grain sizes, micro-cracks, adhesion characteristics and elastic properties may be important for the performance and reliability of the devices, especially when highly scaled microstructure devices are considered. By way of example, in metallization layers of advanced integrated circuits, the adhesion characteristics may significantly affect the mechanical strength during the manufacturing process and may also determine the reliability of the finished device while the crystallinity of the metal also influences the current drive capability and thus the performance of the device. Moreover, the formation of well-understood interfaces between two different materials is an important aspect and may therefore require reliable measurement data of surface characteristics.
Consequently, there was a growing need for techniques which may be able to characterize materials and in particular their engineered surfaces with a spatial resolution that is appropriate for highly scaled microstructures, such as integrated circuits. A frequently used and very powerful tool in this respect is the atomic force microscope (AFM) which allows the characterization of nano-structured materials by scanning an appropriate tip of a cantilever across a sample surface. During the scan operation, typically the charge cloud of the tip interacts with respective charge clouds of the sample surface, wherein the corresponding interaction, i.e., the minimal displacements of the tip, are recorded to obtain information on the surface structure with a nanometer resolution. Thus, atomic force microscopy provides a three-dimensional image of the surface topography, which may provide precious information with respect to the surface structure of the sample. However, when imaging a sample surface with high spatial resolution, respective surface portions within an area of a few micrometers or smaller may appear almost atomically flat so that the corresponding image obtained by means of the atomic force microscope includes less contrast and thus may not allow the extraction of detailed information on surface characteristics at this high resolution, although, on a broader scale, a significant sample topography may be present.
Therefore, recently, a new technique has been developed, in which image contrast and thus extraction of details of small areas of interest may be efficiently enhanced by exciting the sample surface with sound waves, wherein ultrasonic sound with frequencies up to several MHz may be used. A corresponding excitation of the sample surface is typically used in combination with an atomic force microscope and this technique is typically referred to as ultrasonic force microscopy (UFM). Using this technique, the tip of the atomic force microscope is used to detect the acoustic or ultrasonic waves via the sample surface, wherein elastic changes of the surface below the tip permits extraction of detailed information on the elastic properties of the sample surface with high spatial resolution.
With reference to
During the operation of the system 100, the sample 102 is positioned on the substrate holder 101 and is scanned in one or two dimensions, while the displacement of tip 104, biased with a specific force as controlled by the PI controller 106, is detected by the detector 108, the signal of which is processed by the lock-in amplifier 109. During the measurement procedure, a specific modulation signal 112 is supplied to the substrate holder 101 to excite the sample, and thus the sample surface, with a specified signal containing frequencies up to several MHz, wherein these frequencies may be modulated with a corresponding carrier wave, such as a sawtooth wave and the like. During the scan operation, the output signal of the detector 108 is picked up by the lock-in amplifier 109, which is tuned to the modulation frequency, thereby obtaining information on the power of a modulation frequency component in the signal spectrum provided by the detector 108. This signal may further be used by the data processor 110 to provide an image, that is, a qualitative mapping of the UFM response to exciting the sample 102, with a high spatial resolution compared to pure AFM techniques. Consequently, surface characteristics, such as crystallinity, elastic properties and the like, may be qualitatively investigated and monitored with high spatial resolution.
It appears, however, that for a quantitative determination of specific nanomechanical properties, such as highly precise values of elastic properties of surface portions, the conventional UFM system 100 may suffer from a plurality of problems. For instance, the UFM signal may be strongly modified by signal artifacts, which may originate from the PI controller 106 that provides the bias force of the tip 104 during the scan operation. Since the lock-in amplifier 109 is tuned to the modulation frequency, such substantially DC or low frequency interferences may not be identified by the data processor 110, thereby resulting in highly unreliable measurement data. Moreover, the data processor 110 receives information about the power of a modulation frequency component but does not receive any information about the waveform of the signal itself. Consequently, in sophisticated applications, a precise and reliable determination of surface-related characteristics may be difficult with conventional systems, even if a conventional UFM tool is capable of providing a high spatial resolution.
In view of the above-described situation, a need exists for an enhanced technique that avoids or at least reduces one or more of the problems identified above.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.
Generally, the present invention is directed to a technique that enables the extraction of information obtained from scanning probe microscope type tools (“SPM-type tools”) operated on the basis of a modulated sample excitation in a more reliable and more complete fashion by using the waveform of the signal as obtained from the SPM tool substantially without data reduction, which may include precious information that is conventionally lost by the pre-processing by means of a lock-in amplifier in a conventional UFM regime. Thus, a more complete data set is created so as to overcome information limits about the nanomechanical properties of the materials based on SPM-type measurements combined with modulation, such as acoustic or ultrasonic modulation. Consequently, the technique according to the present invention is highly advantageous in quantifying nanomechanical properties of microstructures, such as surface characteristics in advanced integrated circuits.
According to one illustrative embodiment of the present invention, a metrology system comprises a scanning probe microscope and a modulation unit connected to the scanning probe microscope to enable a modulated excitation of a sample. Moreover, the metrology system comprises a signal processing unit connected to the scanning probe microscope to receive an output signal therefrom for a plurality of scan positions, wherein the signal processing unit is configured to digitize and record a plurality of frequency components for each scan position. In one illustrative embodiment, the signal processing unit is configured to digitize and record the substantially complete output signal as received, that is, the complete waveform in the time domain is digitized and recorded.
According to another illustrative embodiment of the present invention, a method comprises scanning a sample surface by a scanning probe at at least a plurality of scan positions to obtain a surface-related output signal at each of the plurality of scan positions. Moreover, the sample surface is excited with a modulated signal and the output signal, including a plurality of frequency components, is digitized for each of the plurality of scan positions.
The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
The present invention will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.
Generally, the present invention is directed to a technique that enables the determination of quantitative estimates for a characteristic of surface portions with high spatial resolution in that an increased amount of information is used for the signal processing of an output signal of the SPM tool. For this purpose, in some illustrative embodiments, substantially the full signal obtained from the SPM-like tool may be used, that is, may be digitized and recorded, so as to be available for further data processing, which may be performed on the basis of any appropriate model at any convenient time. The signal may be digitized and recorded substantially in real time so that, for each specified scan position, a complete or substantially complete record of information regarding these specific scan positions is obtained. Based on this measurement data, which may contain a significantly increased amount of information compared to conventional systems that are related to a single frequency component for each scan position due to the application of a lock-in technique, a more efficient data evaluation may be performed on the basis of one or more appropriate analysis algorithms so as to create meaningful quantitative values for a material characteristic of interest. Moreover, since the measurement data recorded for each scan position may also include any DC and low frequency “interferences,” corresponding signal artifacts that may remain undetected in the conventional technique, may be identified and compensated for by appropriately selected mechanisms.
With reference to
The metrology tool 200 may further comprise a modulation unit 203, which may be configured so as to at least excite a surface of the sample 202 in a modulated manner, for instance by mechanically exciting the substrate holder 201, as is described with reference to
In some embodiments, a pulsed laser source may be provided, which may introduce energy into specific portions of the sample 202 in a modulated fashion, that is, in a pulsed fashion. In still other embodiments, the controller 206 and the holder 205, in combination with the tip 204, may be configured to receive the modulated signal from the modulation unit 203 and to excite the sample surface by means of the tip 204.
The metrology system 200 may further comprise a signal processing unit 220 that is configured to receive the output signal of the detector 208 and to provide a digitized value of the output signal for each scan position, thereby providing “real-time” digitizing of the output signal of the detector 208. It should be appreciated that the term “real-time” in this respect is to be understood such that the signal processing unit 220 is capable of at least digitizing a plurality of frequency components for a given scan position such that the original waveform of the output signal is represented by the plurality of frequency components to facilitate a quantitative estimation with a desired precision. For example, digital oscilloscopes provide a sufficiently high sampling rate so as to detect frequency components up to several tenths of MHz, wherein a possible signal distortion due to the limited bandwidth of the oscilloscope may be tolerable with respect to a subsequent data processing on the basis of digitized data sets for each of the plurality of scan positions that are distorted or restricted due to the limited bandwidth. Thus, the term “a plurality of frequency components” is to be understood such that at least a bandwidth is covered that allows estimation of the “true” or original waveform in this frequency band. Moreover, “real-time” processing in this respect is to be understood such that a complete output signal train of the detector 208 is digitized for a specific scan position prior to digitizing a subsequent signal train for a further scan position. In one illustrative embodiment, the signal processing unit 220 is configured to receive the output signal containing the “full” information, except for any signal distortions caused by a restricted bandwidth of the communication path and the input of the signal processing unit 220, as a signal represented in the time domain, wherein the complete waveform is digitized and recorded, thereby maintaining the full amount of information in said output signal. The signal processing unit 220 may comprise an input amplifier 220, which may have, contrary to any extreme frequency selective amplifiers used in conventional SPM systems, a sufficient substantially linear gain characteristic so as to not unduly distort the output signal, at least within a specified frequency range. In some illustrative embodiments, the signal process unit 220 is configured to process DC and low frequency components that may be contained in the output signal of the detector 208. Hence, the input amplifier 221, if provided, may be configured to also amplify respective DC and low frequency components.
The system 200 may further comprise a data memory 223 connected to the signal processing unit 220 in order to store the digitized data received from the signal processing unit 220 as respective output data, wherein the data memory 223 is configured to maintain the spatial correlation of the output data and a respective scan position. Moreover, in some illustrative embodiments, the data memory 223 may also be connected to the modulation unit 203 and may be configured to store the output data in a correlated manner with respect to the modulation frequency used in the modulation unit 203. In some illustrative embodiments, the system 200 may further comprise a lock-in amplifier 209 that may be connected for receiving the output signal from the detector 208 substantially without generating any signal distortion for the input signal of the signal processing unit 220. The lock-in amplifier 209 may be tuned to the modulation frequency of the modulation unit 203, thereby providing real-time data of reduced information contents, which may be advantageous in qualitatively estimating specific sample characteristics. Based on this data of reduced information, a further data manipulation, such as the selection of specific sampling regions, regions of interest and the like, may be performed. Moreover, the metrology system 200 may further comprise a data processor 230 which may have implemented therein one or more evaluation algorithms or models 231 for operating on the output data stored in the data memory 223. In illustrative embodiments, one or more evaluation algorithms 231 may include error compensating mechanisms so as to efficiently identify systematic errors, such as a baseline drift of the controller 206 and the like. Moreover, the metrology system 200 may comprise a data indicator 211 in the form of a display and the like to present the output data manipulated by the data processor 230 in any appropriate form that enables the quantitative estimation of at least one surface characteristic of the sample 202.
During the operation of the metrology system 200, a modulated surface excitation is created for the sample 202 while it is sampled by the tip 204. The output signal of the detector 208 is then received by the signal processing unit 220, which digitizes, in illustrative embodiments, substantially the full output signal including a plurality of frequency components, which may include any DC and low frequency components. In other embodiments, appropriate filter means (not shown) may be used to restrict the output signal of the detector 208 to an appropriate operating range of the signal processing unit 220, wherein, nevertheless, a moderately large bandwidth is provided to estimate the waveform of the output signal of the detector 208. In some illustrative embodiments, a plurality of analog-to-digital converters may be used with different sample and hold times, thereby providing a different degree of “averaging” of the incoming output signal, wherein, for instance, a digitized data with less resolution may be obtained with low delay and may therefore be subjected to a real-time data processing to allow immediate qualitative evaluation of the incoming output signal for each of the plurality of scan positions.
The digitized data provided by the signal processing unit 220, which represent the output signal 208 for each scan position at least within a moderately broad bandwidth, are then stored in the data memory 223 in such a manner that a correlation between a respective scan position and an output signal of the detector 208 associated therewith is maintained. In some illustrative embodiments, information from the modulation unit 203 may be stored in the data memory to correlate the output data with modulation-specific information, which may enable tuning the corresponding output data with respect to, for instance, the modulation frequency in a subsequent data processing operation. Depending on the computational capability of the data processor 230 during and/or after scanning across the sample surface, the data processor 230 may access the data memory 223 and retrieve corresponding output data that may be manipulated in accordance with one or more of the specific models or algorithms 231 for extracting quantitative estimations of one or more surface characteristics of the sample 202. Due to the fact that the waveforms themselves of the output signal delivered by the detector 208 are sampled and stored in the data memory 223, the full or a significantly increased fraction of the information compared to the conventional lock-in based analysis is available and may be exploited in accordance with any appropriate data manipulation algorithm provided in the data processor 230. For example, any edge recognition techniques may be used to operate on the output data so as to identify surface grain boundaries, at which a significant discrepancy of surface elasticity may occur. Due to the increased amount of information available, a corresponding enhanced “contrast” for a two-dimensional image of the scanned area of the sample 202 may be obtained, thereby significantly enhancing the efficiency of a corresponding evaluation algorithm. In other illustrative embodiments, output data correlated with two or more scan positions, such as spatially adjacent scan positions, may be processed to enhance the statistical relevance and reduce the variance of the corresponding evaluated surface characteristic. In other embodiments, the data processor 230 may be configured to enable the definition of one or more regions of interest, i.e., of a plurality of scan positions or pixels in a corresponding image of the sampled surface, wherein in each of the one or more regions of interest a specific data manipulation is commonly provided. For instance, respective output data belonging to a specific one of the one or more regions of interest may appropriately be averaged so as to reduce statistical fluctuations.
According to box 320, the response of the sample surface is detected by the scanning probe and is converted into a respective output signal, for instance by optical means, such as the laser 207 and the detector 208, or any other appropriate means, wherein the output signal is received for a selected scan position.
According to box 330, the output signal is digitized, wherein substantially the full output signal is used so as to suppress any information loss in the subsequent signal processing sequence. Moreover, the process of digitizing the output signal is performed in a “realtime” fashion, that is, the correlation between the output signal digitized and the selected scan position is maintained.
According to box 340, the digitized output data are stored and thus are available for a further signal processing at any later stage, irrespective of the capability of a data processor or any evaluation model or algorithm. Since substantially the full information is available in the stored output data, any iterative or cross-related evaluation techniques may be used, in which, for example, the evaluation result in one model may be used for appropriately selecting further evaluation criteria and performing a corresponding data analysis based thereon.
In box 350, it is decided whether or not a new scan position is to be selected. If the scan area is not yet completely scanned, the process flow returns to box 310 and repeats the steps 320, 330 and 340 to store a further output data set associated with the new scan position. If all desired scan positions are processed, the process flow may advance to box 360, in which it is decided whether a data processing may be performed on the stored digitized output data. Due to the availability of substantially a full information content of the originally detected output signal, very powerful techniques, such as imaging processing techniques, may be used, wherein even external data processors may be used. Thus, if a data processing is not intended at this stage due to non-availability of external resources and the like, the process flow may advance to step 370. If a data processing is desired, in step 380, a desired model or algorithm may be selected and, according to step 390, the data processing may be performed on the basis of the selected algorithm and on the basis of some or all of the output data stored.
As a result, the present invention provides an enhanced technique for the investigation of material characteristics on a nanometer scale, wherein particularly surface-related characteristics may be detected such that quantitative and meaningful values may be determined by using a high amount of information provided by an SPM-type tool operating on an excited sample surface. This is achieved by digitizing the output signal prior to significantly reducing the output signal, as is the case in conventional lock-in techniques, where the data evaluation is reduced to only one given frequency component. Consequently, any type of evaluation algorithm or model may be used for operating on the digitized output data, wherein even complex and powerful algorithms may be used substantially without any restriction with respect to a narrow bandwidth of the output data. In this way, certain characteristics of the measurement process or of the sample may be directly visualized, such as a change of the bias power of the scanning probe. Hence, measurement values that quantify certain surface-related material characteristics may be compared and thus used for classifying these characteristics with high accuracy, thereby providing the possibility to enhance materials and processes for the fabrication of microstructures, such as integrated circuits.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.